# What precisely was the character of the non-gravitational acceleration of Oumuamua?

When exactly did Oumuamua accelerate, when did it begin, when did it end, was it uniform across that period, and at what vector did it occur?

I've seen some vague references to it lasting 100 days, but I can't track down any of this in precision.

## 1 Answer

There is a paper on this subject that you can read here. Basically by fitting the data to a pure gravity simulation, there was some residuals, but when fitting it to the model $$A_1r^2$$, they could find a solution that seems to fit quite well. In this case, $$r$$ is the distance from the sun, measured in AU. The $$A_1$$ value that best fit is $$5.01\cdot 10^{-6} \frac {m}{s^2}$$, with some small uncertainty about that. The difference in these two models can be seen in this figure from the paper. The left is pure gravity, the right with the model indicated here. The Y axis is how much of an error there is in the predicted locations vs what the model predicts. Note that the right ones are much closer to 0, and thus better predict the location.

There are two plausible explanations for this. The paper says the most likely is in the form of a comet, but that is unusual because the shape didn't change, and there was no tail observed. However, the acceleration is very slight, and it would be below the detection levels, barely, meaning this is the most likely explanation.

The second is that it is a very light reflective surface pointed directly at the sun, basically what one would need for this to be a solar sail. If this were to be the case, it would have to be of a very low density.

In any case, it is quite curious, but there isn't really a way to figure out for sure what is going on with this object.

Also of some note is there are quite a few models that fit the data reasonably accurately. What seems reasonably certain is there was a radial acceleration, and not one along the direction of motion.

See also this video I did on the subject.

Lastly, one common misconception is the light curve didn't reveal any change in the rate of motion. It turns out that there were several different period of rotations measured, with the most likely explanation that it was changing with time. See this paper.

• If there were outgassing, this would change the character of the tumbling, right? How could it not? And my understanding is that that was not indicated by the albedo data. Doesn't this alone categorically rule out outgassing? – Chris B. Behrens Jan 14 '19 at 18:36
• So help me with the residuals here...if I'm reading the left chart correctly, the acceleration was NOT uniform, but followed a kind of sine wave? The high positive residuals at 1.24 AU down to low negatives at 1.89 AU, and then back to high positives near 2.5? – Chris B. Behrens Jan 14 '19 at 18:58
• @ChrisB.Behrens "If there were outgassing, this would change the character of the tumbling, right? How could it not?" That's the heart of the problem. If you can find a shape and a motion pattern that matches the brightness and allows for outgassing without tangential acceleration, I think you'd get quite some attention in the respective community. But then you'd still have to explain the absence of detectable gas & particles. – Everyday Astronaut Jan 14 '19 at 19:31
• @PearsonArtPhoto - awesome video! I committed an act of journalism and reached out to Marco Micheli, and he confirmed that at the point where the acceleration occurred, it was too distant to measure the light curve, and only position could be measured. To me, this pushes me strongly over into the outgassing camp. – Chris B. Behrens Jan 17 '19 at 15:29
• I actually found another paper that shows that outgassing is even more likely. See arxiv.org/ftp/arxiv/papers/1712/1712.00437.pdf . Bottom line is, the period of rotation was observed to be changing with time. – PearsonArtPhoto Jan 17 '19 at 16:18